                The following references describe neutrophil elastase inhibitors with a 2-pyridone central core: WO04043924, WO05026123, WO05026124, WO06098683, WO06098684, WO07129962, WO10094964, WO11039528.        The following references describe neutrophil elastase inhibitors with a 2-pyrazinone central core: WO07129963, WO09061271, WO09058076, WO11110852.        The following references describe neutrophil elastase inhibitors with a 4-pyridone central core: WO14029830, WO14029831, WO14029832.        For a review on various inhibitors of neutrophil elastase see: P. Sjo (Future Med. Chem. 2012, 4, 651-660).        